The present invention relates to a gray balance correcting method for correcting the gray balance of an image recorded on a negative film.
An image recorded on a color negative film is pervious to three colors of light; namely, B (blue), G (green) and R (red). It is known, experimentally, that the transmission ratio of the three colors is generally, substantially equal or predetermined (Evans theory). Therefore, in most photographic printers, the exposure condition is determined on the basis of the following equation called an integral neutral method: EQU log Ej=Kj+Aj.multidot.Dj (1)
in which log Ej represents a logarithm of exposure, Kj represents a constant based on a photosensitive material, a photographic printer, etc., Dj represents an accumulated transmission density of the image (LATD), Aj represents a correction coefficient, and j represents a color (R, G or B) of light. According to the aforementioned equation (1), for example, the exposure of R is set large for an image with a small amount of transmitted light (high in the accumulated transmission density of R), so that the total amount of each of the R, G and B light radiated onto print paper, through a negative film, is made coincident with each other when printing on the print paper is based on the exposure calculated as described above. Accordingly, the accumulated density values of R, G and B in an image printed on the print paper are made constant so that the gray balance is maintained over the entire image.
In the case of a negative film, the densities of R, G and B, which develop Magenta (M), Yellow (Y) and Cyan (C), respectively, theoretically change with a constant difference in density, as shown in FIG. 1(A), when the amount of exposure changes at the time of photographing. In the aforementioned integral neutral method, when, for example, accumulated transmission densities are M.sub.0, Y.sub.0 and C.sub.0, exposure is determined to make these coincident with each other. Accordingly, exposure can be determined, theoretically, to make the density change characteristics of the respective colors coincident with each other, so that gray balance ought to be maintained within a range between the minimum density and the maximum density in the printed image. The abscissa of FIG. 1 shows the exposure quantity in log scale.
In practice, however, the density change characteristics of the respective colors in the negative film are affected by processing conditions such as development, etc., so that the slopes of the curves of the density change characteristics are different, as shown in FIG. 1(B). In the case where the slopes of the curves for the density change characteristics for the respective colors are different, displacements in these curves occur in a region (for example, region A shown in FIG. 1(C)) in which the densities are higher than the predetermined values as compared with the accumulated transmission densities. Similar behavior results for a region (for example, region B shown in FIG. 1(C)) in which the densities are lower than the predetermined values. As a result, gray balance is not maintained, and the undesirable situation arises where the colors in the portions of the image that correspond to the aforementioned regions of the image printed on the print paper, are different from the colors of the subject at the time of photographing.
Density failure occurs in situations where the background portion of an image has an extremely high or low density level. In such situations the accumulated transmission density differs greatly from the density of the main subject portion of the image. This is called density failure. When the aforementioned image is printed on print paper, where the basis of exposure is determined by applying the aforementioned integral neutral method, the image is affected by the density failure so that the exposure of the main subject portion of the image becomes either short or excessive.
Color failure occurs in situations where the background portion of an image is wide and is comprised of a specific color, such as green grass, or blue sea, etc., and greatly differs from the color of the main subject. In such a situation the accumulated transmission density of the specific color is high. The result is that the color balance across the entire scene is greatly biassed against the color balance of the main subject. This is called color failure. When the aforementioned image is printed on print paper with the basis of exposure determined by applying the integral neutral method, the image is affected by the color failure so that the color balance of the main subject portion in the image is not maintained. This problem also occurs when the accumulated transmission density of equation (1) is replaced by the average density of the entire scene.
However, in a conventional printer, density failure and color failure are prevented by a so-called lowered correction, where an exposure correction value is lowered. However, in the lowered correction remedy, the color balance correction capacity is reduced with respect to an image photographed by a specific light source other than sun light (such as a fluorescent lamp, a tungsten lamp, etc.), so that an appropriate print result cannot be obtained.
Further, a method of determining exposure by calculating the accumulated transmission density (or average density) over an entire image while removing data from a portion of the image predicted to be highly saturated (see Japanese Patent Postexamination Publication No. Sho-59-29847); a method of determining exposure after converting high-saturation measured light data into achromatic color (see Japanese Patent Unexamined Publication No. Sho-59-29847); and so on, have been proposed. It is however difficult in practice, to judge accurately whether the image is a high-saturation image or whether the image is an image photographed by a specific light source. Furthermore, these methods are problematic in that they involve complicated procedures.
Given these problems, an object of the present invention is to provide a gray balance correcting method in which the color balance correction capacity, with respect to an image photographed by a specific light source, is prevented from being reduced and in which the influence of failure can be eliminated.